In the optical fiber drawing device that produces an optical fiber, generally, the speed of an optical fiber preform being continuously drawn after heated (namely, drawing speed) in a drawing furnace provided with the optical fiber drawing device is closely related with the quality of an optical fiber produced. Accordingly in the optical fiber drawing device, various improvements for adjusting the drawing speed to an optimum speed have been made to obtain a good quality optical fiber.
Normally, the drawing speed itself is feedback-controlled so that the detected value of an outside diameter of an optical fiber after drawn (bare optical fiber) follows a target value; therefore, the control of the real drawing speed to reach an appropriate speed is indirectly performed by controlling the speed (hereafter, called preform feeding speed) of feeding the optical fiber preform being a drawing object into the drawing furnace and so forth.
In an optical fiber drawing device according to the below patent document 1, for example, a preform feeding speed Vf is calculated by the method shown below; the optical fiber preform is fed into the drawing furnace with the calculated preform feeding speed Vf; and thereby the drawing speed is indirectly controlled to reach an appropriate speed. Hereunder the time t represents a time at an instant of time during drawing, and the drawing speed v(t) represents a real drawing speed at the time t.
In the optical fiber drawing device according to the patent document 1, the speed Vf1 being the base speed of feeding the optical fiber preform into the drawing furnace (hereunder, called base preform feeding speed) is calculated by the following formula (A), based on an outside diameter D of the optical fiber preform (hereunder, called preform diameter), a target diameter d (hereunder, called target fiber diameter) of the optical fiber after drawn (bare optical fiber), and a target drawing speed v1 of the optical fiber. This formula (A) is based on the law of mass conservation being valid between the optical fiber preform supplied and the optical fiber after drawn. Here, the target drawing speed v1 is a drawing speed desirable of acquiring a good quality optical fiber in relation with the coating process being the tail-end process in the steady state operation which is given in advance.Vf1=v1*d2/D2  (A)
Next, a practically set preform feeding speed Vf(t) is calculated by correcting the base preform feeding speed Vf1 calculated by the formula (A), with predetermined corrections (ΔVf1 and ΔVf2). Here, the correction ΔVf1 is calculated by multiplying a difference between the target drawing speed v1 and the real drawing speed v(t) by a predetermined positive coefficient K1. The correction ΔVf2 is calculated by multiplying a variation per a unit time of the real drawing speed v(t), namely, V(t)−V(t−Δt) being an acceleration by a predetermined positive coefficient K2.
The practically set preform feeding speed Vf(t) is calculated by the following formula (B), which subtracts the correction ΔVf2 from the result of adding the correction ΔVf1 to the above base preform feeding speed Vf1.Vf(t)=Vf1+ΔVf1·ΔVf2=v1·d2/D2+K1(v1−V(t))−K2(v(t)−v(t−Δt))  (B)
By setting the preform feeding speed Vf according to this formula (B) the real drawing speed v(t) is indirectly controlled to follow the target drawing speed v1. In concrete, as the real drawing speed v(t) falls below the target drawing speed v1, the preform feeding speed Vf is accelerated according to the formula (B); consequently, the eluted quantity of the optical fiber preform increases, and the diameter (detected value) of the optical fiber after drawn tends to increase; therefore, the real drawing speed v(t) is controlled to increase so that the diameter of the optical fiber approximates to the target diameter d. As the real drawing speed v(t) exceeds the target drawing speed v1, on the contrary, the preform feeding speed Vf is decelerated according to the formula (B); consequently, the real drawing speed v(t) is controlled to decrease so that the diameter of the optical fiber approximates to the target diameter d.
The optical fiber drawing device according to the patent document 1 has a mechanism that drives to rotate a preform feeding motor, so that the speed of feeding the optical fiber preform into the drawing furnace approximates to the preform feeding speed Vf(t) acquired by the above calculation, to thereby produce a good quality optical fiber.
Now, the preform diameter D, target diameter d, and target drawing speed v1 in the steady state operation are preset as the operational conditions of the optical fiber drawing device, and in the optical fiber drawing device according to the patent document 1, as long as the setting of the operational conditions is not modified, the base preform feeding speed Vf1 calculated by the formula (A) is constant.
In contrast, in the unsteady state operation such as a start-up state or terminating state of the optical fiber drawing device, it becomes necessary to adjust the above preform feeding speed by varying it significantly. Accordingly in the unsteady state operation, to rely only on the technique that makes the base preform feeding speed Vf1 constant by the above formula (B) will necessarily correct the preform feeding speed requiring a large adjusting range only by the above correction terms ΔVf1, ΔVf2, which leads to increasing weight in the arithmetic operation of these correction terms.
However, the correction ΔVf1 is a correction term based on a discrepancy of the real drawing speed against the target drawing speed v1; and the correction ΔVf2 is the one related with the dynamic correction, namely, correction of over or short of the correction tendency Therefore, when a required adjusting range is significant, the weight to this correction term becomes huge, which leads to increasing the possibility of creating a malfunction such as hunting or overshoot.
In the optical fiber drawing device according to the patent document 1, it is easy to conceive that the values of the coefficients K1 and K2 being always optimized is the premise for acquiring a precise preform feeding speed Vf(t). However, it is extremely difficult to set the coefficients K1 and K2 as a unique value, which cover the unsteady state such as a start-up state or terminating state of the device, wherein the drawing speed varies significantly. As there occurs a malfunction such as hunting or overshoot from such a cause, the diameter and coating condition of the optical fiber are not stabilized, which causes a quality deterioration or yield lowering of the optical fiber. In addition if the hunting or overshoot generated is excessive, the drawing speed m the optical fiber drawing device will fall into an impossibility of control, which will consequently lead to a breaking of the optical fiber. These problems in the unsteady (transient) state operation such as a start-up state of the optical fiber drawing device are the ones generated in the operational parameters such as the aforementioned drawing speeds and the correction terms, which determine the operational conditions of the optical fiber drawing device.
Further, accompanied with the tendency toward a larger diameter in the optical fiber preform, the start-up fiber length (or speed increasing time) increases, and the lowering of the yield becomes a problem. However, in case of increasing the acceleration in pursuit of shortening the start-up fiber length (or speed increasing time), it will intensify the possibility that causes a malfunction such as hunting or overshoot in the start-up tail end (near the target drawing speed.
Therefore, the present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical fiber drawing device capable of a stable drawing control that does not generate hunting or overshoot or the like, while reducing the start-up fiber length (or speed increasing time) in the unsteady state (transient state) operation of the optical fiber drawing device.    Patent Document 1: Japanese Patent Lad-Open No. Hei10(1998)-81538